Doped carbon nanotubes and transparent conducting films containing the same

ABSTRACT

Transparent conducting electrodes include a doped single walled carbon nanotube film and methods for forming the doped single walled carbon nanotube (SWCNT) by solution processing. The method generally includes depositing single walled carbon nanotubes dispersed in a solvent and a surfactant onto a substrate to form a single walled carbon nanotube film thereon; removing all of the surfactant from the carbon nanotube film; and exposing the single walled carbon nanotube film to a single electron oxidant in a solution such that one electron is transferred from the single walled carbon nanotubes to each molecule of the single electron oxidant.

DOMESTIC PRIORITY

This application is a continuation of and claims priority from U.S.patent application Ser. No. 12/873,427, filed on Sep. 1, 2010, entitled“DOPED CARBON NANOTUBES AND TRANSPARENT CONDUCTING FILMS CONTAINING THESAME,” the entire contents of which are incorporated herein byreference.

BACKGROUND

The present invention relates to doped carbon nanotubes and moreparticularly, to transparent conductive films formed of the doped carbonnanotubes.

Transparent conducting electrodes are key components of many modernelectronic devices including photovoltaic cells, organic light emittingdiodes, flat panel displays and touch screens. The most widely usedtransparent conducting electrode is indium tin oxide (ITO). Despite theexceptional optoelectronic properties of ITO (e.g., sheet resistance of5 to 10 ohms per square (Ω/□) at >85% transmittance), the materialsuffers from considerable drawbacks including increased materials costdue to scarcity of indium and the costs associated with high temperaturevacuum deposition. Additionally, vacuum deposited ITO films are brittleand therefore not suitable for flexible electronics.

It turns out that very thin carbon nanotube films as thin as 10 or 20nanometers are transparent to visible light and can conduct electricity,which makes them candidates for transparent conducting electrodes.Doping of the carbon nanotubes in films can increase overallconductivity. Currently, the most commonly used chemical dopants fornanotube networks are nitric acid and thionyl chloride. Although thesematerials provide excellent doping efficiency, these materials also havedrawbacks. For example, both chemicals are relatively harsh chemicalsand require special handling. Still further, the volatility of thesematerials leads to sheet resistances that are unstable over time andincreases to values approaching those of undoped films.

To address these drawbacks, single electron oxidants have beendeveloped, which can utilize milder solvents. These single electronoxidants dope the nanotubes by removing an electron from the carbonnanotube, presumably forming a stable charge transfer complex. However,these materials have exhibited low doping efficiency. For example, aciddopants have been found to provide 50% more doping efficiency than theprior art single electron oxidant dopants. Moreover, current singleelectron oxidant dopants do not exhibit good stability. Although thecharge transfer complex should have resulted in greater stability, thishas not been observed experimentally.

Accordingly, there is a need for alternative materials for use astransparent conducting electrodes, especially materials that avoid theuse of nitric acid and thionyl chloride as chemical dopants, arenon-volatile, and have a sheet resistance and an optical transmittancesuitable for use as transparent conducting electrodes.

SUMMARY

Aspects of the invention include a method for doping a carbon nanotubeand a transparent conducting electrode. The method comprises depositingsingle walled carbon nanotubes dispersed in a solvent and a surfactantonto a substrate to form a single walled carbon nanotube film thereon;removing all of the surfactant from the carbon nanotube film; andexposing the single walled carbon nanotube film to a single electronoxidant in a solution such that one electron is transferred from thesingle walled carbon nanotubes to each molecule of the single electronoxidant.

The transparent conducting electrode comprises a transparent substrate;and a doped single walled carbon nanotube film deposited thereon,wherein the doped single walled carbon nanotube film is free ofsurfactant and is doped by solution processed doping with a singleelectron oxidant, such that one electron is transferred from the carbonnanotube to each molecule of the single electron oxidant.

Additional features and advantages are realized through the techniquesof the present invention. Other embodiments and aspects of the inventionare described in detail herein and are considered a part of the claimedinvention. For a better understanding of the invention with advantagesand features, refer to the description and to the drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The subject matter that is regarded as the invention is particularlypointed out and distinctly claimed in the claims at the conclusion ofthe specification. The foregoing and other objects, features, andadvantages of the invention are apparent from the following detaileddescription taken in conjunction with the accompanying drawings inwhich:

FIG. 1 graphically illustrates doping stability for single electrondopant with and without removal of surfactant prior to doping;

FIG. 2 graphically illustrates doping efficiency for single electrondopant with and without removal of surfactant prior to doping;

FIG. 3 graphically illustrates transmittance as a function of wavelengthof an undoped single walled carbon nanotube film and a film doped with asingle electron oxidant;

FIG. 4 graphically illustrates percent transmittance as a function ofsheet resistance of an undoped single walled carbon nanotube (SWCNT)film, an as-prepared SWCNT film doped via exposure to ambient dopant,and a film doped with a single electron oxidant;

FIG. 5 graphically illustrates transmittance as a function of time for aprior art nitric acid doped SWCNT film, and a single electron oxidantdoped SWCNT film prepared in accordance with the present invention,respectively; and

FIG. 6 graphically illustrates percent transmittance at the S22 peak fora single electron oxidant doped SWCNT film compared to a nitric aciddoped SWCNT film; and the ratio of dc conductivity (α) and opticaladsorption coefficient (σ) as a function of time for a prior art nitricacid doped SWCNT film and a single electron oxidant doped SWCNT filmprepared in accordance with the present invention.

The detailed description explains the preferred embodiments of theinvention, together with advantages and features, by way of example withreference to the drawings.

DETAILED DESCRIPTION

In one embodiment, the present invention relates to transparentconducting electrodes comprising a doped single walled carbon nanotubefilm and a method for forming the doped single walled carbon nanotube(SWCNT) by solution processing. The method generally includes removingsurfactant from a deposited SWCNT film, solution doping thesurfactant-free nanotube film with a single electron oxidant at anelevated temperature, and drying the film to form the SWCNT film. Thesingle electron oxidant forms a stable charge-transfer complex with theSWCNTs, thereby injecting holes into the SWCNT film. Deposition of thenanotube film is not limited to any particular deposition method andgenerally involves the use of carbon nanotubes dispersed in a solventand a surfactant, e.g., filtration, spray deposition, electrophoresis,drop-drying, direct growth, and the like. By removing the surfactantprior to solution doping, the resulting doped SWCNT films of theinvention are stable over time and exhibit improved performance relativeto acid doped films. Unlike prior art single electron transfer dopednanotubes, the charge transfer complex is stable and non-volatile.

In one embodiment, the surfactant in an as-deposited undoped SWCNT filmis completely removed by exposing the film to an acid for an extendedperiod of time. The surfactants are not intended to be limited. Severalcommercial surfactants, such as sodium dodecyl sulfate, sodium cholate,triphenyl derivatives, and sodium dodecylbenzene sulfonate have beendemonstrated to efficiently disperse bundled single-walled carbonnanotubes into suspensions of individual nanotubes.

Suitable acids include, but are not limited to, sulfuric acid,hydrochloric acid, trifluoromethane sulfonic acid, methane sulfonicacid, and the like. The particular acid, acid strength and time are notintended to be limited. Transmission electron microscopy (TEM) can beemployed to determine whether the surfactant has been completelyremoved. Residual surfactant, if present, can be observed as adhering tothe nanotube bundles. Suitable acids include, without limitation, nitricacid, sulfuric acid, hydrochloric acid, trifluoromethane sulfonic acid,methane sulfonic acid, acetic acid and the like. By way of example, theundoped SWCNT can be exposed to 12 M nitric acid for one hour toeffectively remove the surfactant. After exposure to the strong acid,the films are rinsed with water to remove the acid.

In another embodiment, the surfactant in an as-deposited undoped SWCNTfilm is completely removed by exposing the film to a base for anextended period of time. Suitable bases include, without limitation,sodium hydroxide, potassium hydroxide, ammonium hydroxide, and the like.

In yet another embodiment, the surfactant in an as-deposited undopedSWCNT film is completely removed by exposing the film to a gas at atemperature effective to react with the surfactant; and rinsing thesingle walled carbon nanotube film with deionized water. Suitable gasesfor the gas phase reaction include without limitation, oxygen, hydrogen,argon, and the like. During the gas phase reaction, the temperature isgenerally maintained within a range of about 80 to about 450° C.

In still other embodiment, the surfactant can be removed by exposure toan oxidant or a reductant that effectively reacts with the surfactant torender the surfactant rinse removable. Suitable oxidants include,without limitation, potassium permanganate, hydrogen peroxide, periodicacid, osmium tetroxide, and the like. Suitable reductants include,without limitation, hydrazine, phenylhydrazine, ammonia, and the like.

In still another embodiment, the surfactant is removed by organicsolvents, not limited to, alcohols such as ethanol, methanol, and thelike; halogenated organic solvents such as methylene chloride,chloroform, and the like; and polar aprotic solvents such as dimethylformamide, n-methylpyrolidone, acetonitrile, dimethyl sulfoxide, and thelike

Once the surfactant has been removed, the SWCNT film can then beintroduced to a doping solution include a single-electron oxidant thatcan oxidize aromatic compounds of the single walled carbon nanotube.P-doping may be accomplished by the reaction of the single electronoxidant and aromatic compound on the carbon nanotube resulting in theformation of a charge transfer complex. The interaction of the carbonnanotube with the charge transfer complex results in the formation of acharged (radical cation) moiety on the carbon nanotube. In oneembodiment, doping with the single electron oxidant is performedimmediately after removal of the surfactant with the acid and subsequentrinsing. In other embodiments, the acid cleaned films are immersed indeionized water to avoid drying of acid residues. The acid cleaned filmsare then rinsed with an organic solvent such as acetone, dichloroethane,and the like and then dried. The dried acid cleaned films can then bedoped in the same manner as discussed above.

Solution processed doping with the single electron oxidant is carriedout in an organic solvent including, without limitation,dichloromethane, acetonitrile, chloroform and mixtures thereof. Examplesof single electron oxidants useful in the solution processed doping ofthe invention include, without limitation, organic single electronoxidants, metal organic complexes pi-electron acceptors, and silversalts. Examples of organic single electron oxidants include antimonycompounds such as trialkyloxonium hexachlroantimonate, antimonypentachloride, nitrosonium salts (for example triethyl oxoniumtetrafluoroborate), tris-(pentafluorophenyl) borane and nitrosoniumcation. Examples of metal organic complexes includetris-(2,2′-bipyridyl) cobalt (III) and tris-(2,2′-bipyridyl) ruthenium(II). Examples of pi electron acceptors includetetracyanoquinodimethane, benzoquinone, tetrachlorobenzoquinone,tetraflurobenzoquinone, tetracynaoethylene,tetrafluoro-tertracyanoquinodimethane, chloranil, tromanil anddichlorodicyanobenzoquinone. Examples of silver salts include silverfluoride, and silver trifluoroacetate. For organometallic dopants,common organic solvents like acetonitrile, tetrahydrofuran and aromatichydrocarbons and chlorinated solvents like dichloromethane andchloroform are suitable. For inorganic salts like silver fluoride eitheralcohols or mixture of alcohols and water can be employed.

In one embodiment, triethyloxonium hexachloroantimonate (C2H5)3O+SbCl6−can be used, e.g., with concentration range of about 1 to about 20 mM,and temperature range of about 10 to about 100° C., and one exemplarysolvent is acetonitrile. In other embodiments, the temperature is about30 to 100° C., and in still other embodiments, the temperature is about70 to about 80° C. It is believed that the antimonite reacts as followswith the carbon nanotube. If 1 represents the benzene ring(s) on acarbon nanotube, then2[1]+3[(C2H5)3O+SbCl6−]---→2[1+.SbCl6−]+3C2H5Cl+3(C2H5)2O+SbCl3. Thedoped nanotubes are stable in ambient conditions. Any excess singleelectron oxidant on the nanotube is removed by rinsing the nanotube inthe same or different organic solvent used in the doping process.Following rinsing, the film is dried under vacuum.

In one method of the invention, bulk doping is achieved by stirring asuspension of the nanotube in the organic solvent in the presence of thesingle electron oxidant in solution in the organic solvent. The rate ofthe reaction and the temperature are controlled to achieve a chargedensity of from about 0.01 to about 1 electron/nanometer of length oftube. Any excess single electron oxidant on the nanotube is removed byrinsing the nanotube in the same or different organic solvent used inthe doping process. Following rinsing, the sample, e.g., the bulknanotube is dried under vacuum.

The doped single walled carbon nanotube film can be deposited on atransparent substrate to form a transparent conducting electrode. Thesubstrate can be glass or plastic, which may be rigid or flexible. Inone embodiment, the thickness of the doped single walled carbon nanotubefilm is 0.001 microns to 1 microns and in other embodiments, thethickness is 0.01 microns to 0.2 microns and in still other embodiments,the thickness is 0.02 microns to 0.1 microns.

A figure of merit for any transparent conductor is given by thefollowing equation:σ/α=−{R _(s)ln(T+R)}⁻¹,Wherein σ and α are the dc conductivity and optical adsorptioncoefficient, respectively. R_(s) is the nanotube film sheet resistance,T is the film transparency, and R is the film reflectance. In oneembodiment, the doped single walled carbon nanotube film has a σ/α ratioof greater than 0.01, and in other embodiments, the σ/α ratio is greaterthan 0.1. As for sheet resistance, in one embodiment the sheetresistance is less than 1000 Ω/□ to greater than zero, and in otherembodiments, the sheet resistance is less than 100 Ω/□ to greater thanzero.

As discussed above, prior art single electron transfer doped nanotubesdid not include removing the surfactant prior to doping. It has beendiscovered that removing the surfactant prior to doping with the singleelectron oxidant renders the charge transfer complex stable andnon-volatile. FIG. 1 graphically illustrates the effect of removingsodium dodecylsulfate surfactant on doping stability, wherein the dopantwas triethyloxonium hexachloroantimonate single electron dopant Asshown, stability markedly increased as a function of time if thesurfactant was removed prior to doping compared to not removing thesurfactant. FIG. 2 graphically illustrates single electron dopant dopingefficiency with and without removable of the surfactant, which wasmarkedly increased by removing the surfactant compared to not removingthe surfactant prior to doping.

The disclosure is further illustrated by the following non-limitingexamples.

EXAMPLES Example 1

In this example, doped SWCNT films were prepared and compared to undopedfilms. Are discharged single walled carbon nanotubes (SWCNT) werepurchased from Iijin Nanotech and subjected to step gradientcentrifugation to yield a highly purified, concentrated solution ofSWCNTs. Films were fabricated using vacuum filtration. The purifiedSWCNTs were first diluted (1:100 by volume) with 1% sodiumdodecylsulfate surfactant and filtered through a mixed cellulosemembrane (MF-Millipore Membrane, mixed cellulose esters, hydrophllic,0.1 μm, 25 mm) to form a uniform SWCNT film. The volume of SWCNTsolution determines the film thickness. The filtration speed was kept aslow as possible to provide the highest degree of uniformity. The SWCNTfilms were then allowed to set for 15 to 20 minutes, followed by passing50 milliliters of water through the membrane to wash off residualsurfactant. The film was then air dried for 15 minutes.

A glass substrate was wetted with water and the membrane with the SWCNTdown was pressed onto the wetted glass substrate. The membrane was thenslowly dissolved in acetone to leave the nanotube film on the glasssubstrate.

Surfactant was then removed from the nanotube film by exposing the filmto 12M nitric acid for 1 hour followed by rinsing with water.Transmission electron microscopy (TEM) was utilized before and afterexposure to nitric acid to verify complete removal of surfactant.

The nanotube film was then doped with triethyloxoniumhexachloroantimonate from Sigma Aldrich Company. The triethyloxoniumhexachloroantimonate (100 mg) was dissolved in dichlorethane(chloroform, toluene, chlorobenzene, dichlorobenzene etc.) (10 ml) andthe SWCNT films were soaked in the dopant solution for 1 hour at 70° C.The film was then rinsed with acetone to remove excess dopant molecules.

FIG. 3 graphically illustrates UV-VIS-NIR absorption spectra for bothundoped and doped SWCNT films. The undoped films were vacuum annealedprior to the measurement to ensure the removal of unintentional dopants.The spectra revealed three main sets of adsorption bands that areattributed to the 1-D band structure of SWCNTs. The S11 adsorption peakcorresponds to the first set of Van Hoves singularities above and belowthe band gap in semiconducting SWCNTs. Similarly, S22 and M11 peakscorrespond to the second set of Van Hove singularities in semiconductingSWCNTs and the first set in metallic SWCNTs respectively. When the oneelectron oxidizing dopant reacts with the SWCNT, an electron istransferred from the SWCNT. The depletion of electrons from the valanceband leads to an attenuation of the adsorption peaks and is strongevidence of charge-transfer doping. The spectra shows complete andnear-complete attenuation of the S11 and S22 peaks of the semiconductingSWCNTs, respectively. As the electrons are depleted form the nanotubefilm, the hole carrier density increases, leading to effective p-typedoping.

FIG. 4 graphically illustrates transmittance versus sheet resistance forundoped, as prepared, and doped SWCNTs. The sheet resistance wasmeasured using a standard four point probe. The as-prepared films (i.e.,unintentionally doped via exposure to ambient dopants in the air) had aσ/α of 0.055. The undoped films (i.e., vacuum annealed and thenimmediately measured) exhibited an σ/α ratio of 0.032. For the dopedfilms, the σ/α was 0.13. This represents an increase by a factor ofabout 2.4 to 4 relative to the as-prepared and undoped vacuum annealedfilms, respectively. This increase in σ/α is comparable to increasesobserved with concentrated acid baths that are typically employed, i.e.,nitric acid and thionyl chloride. These ratios correspond to films (witha % T of 75% at 550 nm) with an Rs of 246 ohms per square (Ω/□) for theundoped film, 152 Ω/□ for the as-prepared film, and 63 Ω/□ for the oneelectron oxidant doped films.

Example 2

In this example, stability of doped SWCNTs prepared with the singleelectron oxidant in accordance with Example 1 was compared to dopedSWCNTs prepared with nitric acid. FIG. 5 graphically illustratesUV-VIS-NIR spectra taken at various times from immediately after dopingto 98 hours after doping with nitric acid and the single electronoxidant, respectively. As shown, initially both films exhibitedeffective doping with both the S22 and S11 adsorption bands completelyattenuated. However, as the nitric acid desorbs, the S22 transitionsbegin to regain their oscillator strength. As shown, the S22 peaksrapidly increase in intensity as a function of time. Similarly, the S11transitions start to gain strength and continually increase through 98hours after doping with nitric acid. The spectrum taken at 98 hoursreveals an S22 region that has regained virtually all of the originalstrength and a small peak at the S11 region. These changes indicate adrastic reduction in the hole carrier density and a de-doping of thefilm.

In contrast, the single electron oxidant doped film was remarkablystable. The S22 region displays minimal change as a function of time.There was an initial small increase in the S22 peaks that was saturatedover time. The S11 regions exhibit virtually no change after 98 hours ofair exposure. This data implies that he increased hole-carrier densityin the single electron oxidant doped film was stable over time.

FIG. 6 is a plot of transmittance as a function of time at the S22 peakfor the nitric acid doped SWCNT films and the single electron oxidantdoped SWCNT films, respectively. The data was extracted from theadsorption data of FIGS. 3 and 4. A decrease in transmittance representsan increase in the S22 peak height and therefore, d-doping of the film.Although only relative differences are considered here, the two filmshave comparable transparencies (% T of 56.3 and 60.7 at 550 nm fornitric acid and single electron oxidant doped films, respectively). Thesingle electron doped films showed a slight drop during the first fewhours followed by saturation in the peak height over time. In contrastthe nitric acid doped film showed a large drop in the transmittancefollowed by saturation after 24 hours.

FIG. 6 is a plot of σ/α as a function of time for both the nitric aciddoped film and the single electron oxidant doped film. Since thetransparency at 550 nm does not change with time, the change in σ/α isextensively due to changes in the electrical conductivity. The change inσ/α for the single electron oxidant doped film is virtually identical tothat shown in FIG. 6 showing a slight drop initially followed bysaturation. Because of the similar behavior, the optical data has provento be an excellent predictor of the electrical performance of the film.The change in σ/α for the nitric acid doped film also closely followedthat observed optically in FIG. 6 but diverges at 24 hours. While thechange in the S22 peak saturates, the conductivity continued to decreasein the nitric acid doped films. The S22 regained nearly all of theoriginal oscillator strength. Although the S22 peak was saturated after24 hours, the S111 peak begins to increase over time as the filmcontinues to de-dope. The continual de-doping of the film results in α/σdecreasing for the duration of the study. After 98 hours post-doping,σ/α of the nitric acid doped film decreased to a value of 0.05 from0.14, decreasing by a factor of 3. Referring back to FIG. 4, this valueis comparable to that observed with the as prepared films that weredoped as a function of exposure to ambient dopants in the air. In thecase of the nitric acid doped films, the doping and gains in sheetresistance are no longer observed after a few days. In stark contrast,the σ/α of the single electron oxidant doped films decreased slightly,about 15%, reaching a value of 0.11. This value is still a factor ofabout 2 larger than that of the as prepared films and a factor of about3 larger than that of the undoped films.

It should be noted that the structures as illustrated in the Figures ofthe present invention are not drawn to scale. Namely, the variousstructures are illustrated as exemplary examples. As such, the length,height and width of various structures as shown in the Figures shouldnot be interpreted as a limitation in the present invention.

All ranges disclosed herein are inclusive of the endpoints, and theendpoints are combinable with each other.

All cited patents, patent applications, and other references areincorporated herein by reference in their entirety.

The use of the terms “a” and “an” and “the” and similar referents in thecontext of describing the invention (especially in the context of thefollowing claims) are to be construed to cover both the singular and theplural, unless otherwise indicated herein or clearly contradicted bycontext. Further, it should further be noted that the terms “first,”“second,” and the like herein do not denote any order, quantity, orimportance, but rather are used to distinguish one element from another.

While the preferred embodiment to the invention has been described, itwill be understood that those skilled in the art, both now and in thefuture, may make various improvements and enhancements which fall withinthe scope of the claims which follow. These claims should be construedto maintain the proper protection for the invention first described.

What is claimed is:
 1. A method for making a transparent conductingelectrode, the method comprising: depositing single walled carbonnanotubes dispersed in a solvent and a surfactant onto a substrate toform a single walled carbon nanotube film thereon; removing all of thesurfactant from the single walled carbon nanotube film by acid cleaning;and doping by a solution process comprising exposing the single walledcarbon nanotube film to a single electron oxidant in a solution suchthat one electron is transferred from each of the single walled carbonnanotubes to each molecule of the single electron oxidant to form aplurality of stable charge transfer complexes, and wherein further thedoped single walled carbon nanotube film has a σ/α ratio of greater than0.1.
 2. The method of claim 1, wherein acid cleaning comprises exposingthe single walled carbon nanotube film to an acid at a strength and fora period of time effective to remove the surfactant; and rinsing thesingle walled carbon nanotube film with deionized water.
 3. The methodof claim 1, wherein the concentration of the single electron oxidant isfrom about 0.01 mM to about 20 mM.
 4. The method of claim 1, wherein thesingle electron oxidant is selected from at least one of: organic oneelectron oxidant, metal organic complex, pi-electron acceptor and silversalt.
 5. The method of claim 4, wherein the organic single electronoxidant is selected from at least one of: trialkyloxoniumhexachlroantimonate, antimony pentachloride, nitrosonium salt, triethyloxonium tetrafluoroborate, tris-(pentafluorophenyl) borane andnitrosonium cation.
 6. The method of claim 4, wherein the metal organiccomplex comprises at least one of tris-(2,2′-bipyridyl) cobalt (III) andtris-(2,2′-bipyridyl) ruthenium (II).
 7. The method of claim 4, whereinthe pi electron acceptor is selected from at least one of:tetracyanoquinodimethane, benzoquinone, tetrachlorobenzoquinone,tetraflurobenzoquinone, tetracynacethylene,tetrafluoro-tertracyanoquinodimethane, chloranil, bromanil anddichlorodicyanobenzoquinone.
 8. The method of claim 4, wherein thesilver salt is silver fluoride.
 9. The method of claim 1, whereinexposing the single walled carbon nanotube film to the single electronoxidant in the solution is at a temperature of 10 to 100° C.
 10. Themethod of claim 1, wherein exposing the single walled carbon nanotubefilm to the single electron oxidant in the solution is at a temperatureof 70 to 80° C.
 11. The method of claim 1, further comprising immersingthe doped carbon nanotube film in deionized water subsequent to removingall of the surfactant from the carbon nanotube film and prior toexposing the single walled carbon nanotube film to the single electronoxidant in the solution.
 12. A method for making a transparentconducting electrode, the method comprising: depositing single walledcarbon nanotubes dispersed in a solvent and a surfactant onto a flexibletransparent plastic substrate to form a single walled carbon nanotubefilm thereon; removing all of the surfactant from the single walledcarbon nanotube film by acid cleaning; and doping by a solution processcomprising exposing the single walled carbon nanotube film to a singleelectron oxidant in a solution such that one electron is transferredfrom each of the single walled carbon nanotubes to each molecule of thesingle electron oxidant to form a plurality of stable charge transfercomplexes, and wherein further the doped single walled carbon nanotubefilm has a σ/α ratio of greater than 0.1.
 13. The method of claim 12,wherein the doping is p-doping.
 14. The method of claim 12, wherein thesingle electron oxidant is selected from at least one of:trialkyloxonium hexachlroantimonate, antimony pentachloride, nitrosoniumsalt, triethyl oxonium tetrafluoroborate, tris-(pentafluorophenyl)borane and nitrosonium cation.
 15. The method of claim 12, wherein thedoped single walled carbon nanotube film has a σ/α ratio of greater than0.01.